Cascode switch with robust turn on and turn off

ABSTRACT

A cascode switch includes a first power transistor configured to be coupled to a load and a second power transistor coupled in series with the first power transistor so that the second power transistor is between ground and the first power transistor. The second power transistor is operable to switch on and off responsive to a pulse source coupled to a gate of the second power transistor. The first power transistor is operable to switch on and off responsive to the same pulse source as the second power transistor or a DC source coupled to a gate of the first power transistor. Alternatively or in addition, a transistor device is coupled to the gate of the first power transistor and operable to actively turn off the first power transistor independent of the load current.

TECHNICAL FIELD

The present application relates to switches, in particular cascode switches.

BACKGROUND

Cascode switches are typically designed with two or more MOSFETs (metal oxide semiconductor field effect transistors) or IGBTs (insulated gate bipolar transistors) connected in series. For example in a two transistor cascode switch, one transistor is coupled to the load and the second transistor is coupled in series between the first transistor and ground. The transistors are switched on and off in order to switch the load current as demanded or required. The load voltage is distributed across all of the series connected power transistors included in the cascode switch. For example, two 800V rated MOSFETs may be connected in series for switching a 1000V or greater load.

A prevalent issue with cascode switches is how to drive the high-side MOSFET or IGBT i.e. the transistor coupled closest to the load. In conventional approaches, a high voltage source such as a Zener diode in series with a resistor is used to turn on the high-side transistor, and the load current flowing through the high-side transistor is used to passively turn off the transistor. The low-side transistor i.e. the transistor coupled closest to ground is switched on and off with a pulsed low voltage source. The disadvantage of this approach is that the passive turn off of the high-side transistor is highly dependent on load current, and during turn off of the high-side transistor there is high dissipation on the low-side transistor voltage limiter.

SUMMARY

Disclosed herein are embodiments in which a low voltage source is used to turn on a high-side transistor of a cascode switch. Other embodiments described herein use a local driver stage to actively turn off the high-side transistor of the cascode switch independent of load current. Various ones of these embodiments may be combined with each other as described herein to yield a cascode switch in which a low voltage source is used to turn on the high-side transistor of the switch and the high-side transistor is actively turned off independent of load current. Further embodiments described herein provide for recovery of a portion of the turn off energy used to turn off of the high-side transistor, avalanche protection and lossless switching.

According to an embodiment of a cascode switch, the cascode switch includes a first power transistor configured to be coupled to a load and a second power transistor coupled in series with the first power transistor so that the second power transistor is between ground and the first power transistor. The second power transistor is operable to switch on and off responsive to a pulse source coupled to a gate of the second power transistor, and the first power transistor is operable to switch on and off responsive to the same pulse source as the second power transistor or a DC source coupled to a gate of the first power transistor. A transistor device can also be coupled to the gate of the first power transistor for actively turning off the first power transistor independent of the load current.

According to an embodiment of a circuit, the circuit includes a load operable to draw a load current and a cascode switch operable to switch the load current. The cascode switch includes a first power transistor coupled to the load and a second power transistor in series between ground and the first power transistor. A gate of the second power transistor is coupled to a pulse source and a gate of the first power transistor is coupled to the same pulse source as the second power transistor or a DC source. The circuit further includes a rectifier diode coupling the pulse source or the DC source to the gate of the first power transistor. A transistor device can also be provided for actively turning off the first power transistor independent of the load current.

According to an embodiment of a cascode switch, the cascode switch includes a first power transistor configured to be coupled to a load and a second power transistor in series with the first power transistor so that the second power transistor is between ground and the first power transistor. The cascode switch further includes a device coupled to a gate of the first power transistor and operable to actively turn off the first power transistor independent of the load current.

According to an embodiment of a method of switching a load via a cascode switch having a first power transistor coupled to the load and a second power transistor in series between ground and the first power transistor, the method includes: switching on and off the second power transistor of the cascode switch via a pulse source coupled to a gate of the second power transistor; and switching on and off the first power transistor of the cascode switch via the same pulse source as the second power transistor or via a DC source coupled to a gate of the first power transistor.

According to another embodiment of a method of switching a load via a cascode switch having a first power transistor coupled to the load and a second power transistor in series between ground and the first power transistor, the method includes: switching on the first and second power transistors of the cascode switch; and switching off the first and second power transistors of the cascode switch independent of the load current.

Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. The features of the various illustrated embodiments can be combined unless they exclude each other. Embodiments are depicted in the drawings and are detailed in the description which follows.

FIG. 1 illustrates a block diagram of a circuit including a load and a cascode switch coupled to the load, the cascode switch having a high-side power transistor with a low voltage source input.

FIG. 2 illustrates a block diagram of a circuit including a load, a cascode switch coupled to the load and a device for actively turning off a high-side transistor of the cascode switch independent of load current.

FIG. 3 illustrates a block diagram of a circuit including a load and a cascode switch coupled to the load, the cascode switch having a high-side power transistor and a low-side power transistor coupled to the same low voltage source input.

FIG. 4 illustrates a block diagram of a circuit including a load and a cascode switch coupled to the load, the cascode switch having a high-side power transistor and a low-side power transistor coupled to different low voltage source inputs.

FIG. 5 illustrates a block diagram of a circuit including a load, a cascode switch coupled to the load and an energy recovery device.

FIG. 6 illustrates a block diagram of a circuit including a load, a cascode switch coupled to the load and a bipolar transistor for actively turning off a high-side transistor of the cascode switch independent of load current.

FIG. 7 illustrates a block diagram of a circuit including a load, a cascode switch coupled to the load and bipolar transistors for actively turning off multiple power transistors of the cascode switch independent of load current.

FIG. 8 illustrates a block diagram of a circuit including a load, a cascode switch coupled to the load and voltage limiting circuitry for the cascode switch.

FIG. 9 illustrates a block diagram of a circuit including a load and a cascode switch coupled to the load and having lossless switching.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment of a cascode switch 100 coupled to a load 110. The load 110 is also connected to an input source 120. The cascode switch 100 switches the load current, and includes a first power transistor 102 coupled to the load 110 and a second power transistor 104 coupled in series with the first power transistor 102 so that the second power transistor 104 is between ground and the first power transistor 102. In one embodiment, the cascoded transistors 102, 104 are power MOSFETs. In another embodiment, the cascoded transistors 102, 104 are IGBTs. In each case, the second power transistor 104 (also referred to herein as the low-side transistor or switch) switches on and off responsive to a pulse low voltage source 130 coupled to the gate of the low-side transistor 104.

The first power transistor 102 (also referred to herein as the high-side transistor or switch) switches on and off responsive to the same pulse source as the second power transistor or a DC source 130 coupled to the gate of the high-side transistor 102. In the first case, both transistors 102, 104 of the cascode switch 100 are driven from the same pulse source e.g. as provided by a control circuit. In the second case, the low-side transistor 104 is driven from the pulse source and the high-side transistor 102 is driven from a DC supply voltage e.g. also provided by the control circuit. The second scenario where the high-side transistor 102 is driven from a DC supply voltage may be more favorable for high voltage applications e.g. 1000V and above. In either case, a high voltage source is not used to switch on the high-side transistor 102 according to this embodiment. Instead, low voltage turn on drive is used for both the high-side and low-side transistors 102, 104 of the cascode switch 100.

FIG. 2 illustrates another embodiment of a cascode switch 200 coupled to a load 110. The cascode switch 200 has the same series arrangement of power transistors 102, 104 as in FIG. 1, however the high-side transistor 102 is coupled to a local driver stage 202 which actively turns off the high-side transistor 102 independent of load current. The high-side and low-side power transistors 102, 104 can each have a low voltage input source 130 as described above with reference to FIG. 1, or alternatively the high-side transistor 102 instead may have a high voltage input source. The high-side transistor 102 is actively switched off independent of load current via the local driver stage 202. The embodiments illustrated in FIGS. 1 and 2 may be combined to yield a cascode switch in which a low voltage source is used to turn on the high-side transistor 102 and the high-side transistor 102 is also actively turned off independent of load current.

FIG. 3 illustrates an embodiment of a cascode switch 300 coupled to a load. The load is transformer according to this embodiment, having a primary winding (P1) and a secondary winding (S1). The primary winding P1 is connected to an input source V2 and the secondary winding S1 is connected to a load represented by voltage V3 via a rectifier diode D3. The cascode switch 300 includes a high-side MOSFET Q2 coupled to the primary winding P1 and a low-side MOSFET Q1 coupled in series between ground and the high-side MOSFET Q1. According to this embodiment, the gate of the low-side MOSFET Q1 is coupled to a pulse low voltage source V1 via a resistor R1. The gate of the high-side MOSFET Q2 is coupled to the same pulse low voltage source V1 as the low-side MOSFET Q1, via a resistor R2 and a rectifier diode D4. The cathode of rectifier diode D4 is connected to resistor R2 and the anode is connected to pulse source V1. Power transistors Q1 and Q2 of the cascode switch 300 are driven from the same low voltage pulse source V1 according to this embodiment.

The cascode switch 300 shown in FIG. 3 further includes a diode D2 which functions as a voltage limiter for maintaining the gate-to-source voltage (Vgs) of the high-side MOSFET Q2 below a safe level e.g. 20V. Usually the amplitude of the pulse low voltage source V1 is lower than this safe level (20V in this example) and therefore voltage limiter diode D2 is provided as a safety measure. The gate of the high-side MOSFET Q2 is also coupled to the anode of rectifier diode D5, the cathode of which is connected to the cathode of transient voltage suppression (TVS) diode D1. The anode of TVS diode D1 is connected to ground. TVS diode D1 functions as a high voltage Zener diode.

During turn on of the cascode switch 300, pulse source V1 turns on low-side MOSFET Q1 and rectifier diode D4 begins to conduct. In the conducting state rectifier diode D4 charges the input capacitance of high-side MOSFET Q2 through resistor R2, turning on high-side MOSFET Q2.

During turn off of the cascode switch 300, pulse source V1 turns off low-side MOSFET Q1 which in turn causes the drain-to-source voltage (Vds) of Q1 to begin rising. When Vds of MOSFET Q1 reaches a threshold voltage of TVS diode D1, rectifier diode D5 begins to conduct and load current shifts from the low-side MOSFET Q1 to TVS diode D1. Load current goes through the gate-to-source capacitor Cgs of the high-side MOSFET Q2, discharging Cgs. The high-side MOSFET Q2 turns off when Cgs is discharged. As such, turn off of the high-side MOSFET Q2 depends on the load current according to this embodiment. When Vgs reaches −Vf (forward voltage) of voltage limiter diode D2, current shifts from Cgs to diode D2. Current flows through voltage limiter diode D2 until the high-side MOSFET Q2 completely turns off.

FIG. 4 illustrates an embodiment of a cascode switch 400 which is similar to the embodiment shown in FIG. 3, however the low-side MOSFET Q2 is driven from the pulse source V1 and high-side MOSFET Q1 is driven from a DC source V4. The embodiment shown in FIG. 4 may be more favorable for high voltage applications e.g. 1000V and above.

Described next are additional embodiments of a cascode switch. For ease of illustration, the high-side and low-side transistors of the cascode switch are shown connected to the same low voltage pulse source in each of these embodiments. However, the high-side and low-side transistors can be connected to different input sources as described above.

FIG. 5 illustrates an embodiment of a cascode switch 500 which is similar to the embodiment shown in FIG. 3, however the cathode of rectifier diode D5 is coupled to an energy recovery circuit 510 instead of a TVS diode. The energy recovery circuit 510 includes capacitors C1 and C2, and rectifier diodes D1, D10 and D11. The energy recovery circuit 510 stores energy from the load during switching off of the cascode power transistors Q1 and Q2, and reuses the stored energy to power the load during a subsequent switching cycle of the power transistors Q1 and Q2. The circuit shown in FIG. 5 also has a full wave rectifier 520 including diodes D6, D7, D8 and D9 for rectifying AC input source V2 toward the load.

During turn on of the cascode switch 500, pulse source V1 turns on the low-side MOSFET Q1 and rectifier diode D4 begins to conduct, charging the input capacitance of the high-side MOSFET Q2 through resistor R2. The high-side MOSFET Q2 turns on in response. Voltage limiter diode D2 maintains Vgs of the high-side MOSFET Q2 below a safe level as previously described herein.

During turn off of the cascode switch 500, pulse source V1 turns off the low-side MOSFET Q1 and Vds of Q1 correspondingly begins to rise. When Vds of the low-side MOSFET Q1 reaches half of the bus voltage (i.e. half of the voltage across C1, D10, C2 or in other words the rectified input voltage), rectifier diode D5 begins to conduct and load current shifts from the low-side MOSFET Q1 to the input capacitor C1 of the energy recovery circuit 510. Load current also goes through Cgs of the high-side MOSFET Q2. When Cgs of the high-side MOSFET Q2 is discharged responsive to the load current, the high-side MOSFET Q2 turns off. When Vgs of the high-side MOSFET Q2 reaches −Vf of the voltage limiter diode D2, current shifts from Cgs to the voltage limiter diode D2. Current flows through diode D2 until the high-side MOSFET Q2 completely turns off. Instead of dissipating energy on a TVS diode as is done with the embodiments illustrated in FIGS. 3 and 4, the energy is contained in capacitor C1 of the energy recovery circuit 510 and reused in the next switching cycle of the cascode switch 500 to improve efficiency. Diodes D1, D10, D11 together with capacitors C1 and C2 form a passive factor correction circuit which recovers part of the switching energy.

FIG. 6 illustrates another embodiment of a cascode switch 600 which is similar to the embodiment shown in FIG. 3, however an additional device is provided which is coupled to the gate of the high-side MOSFET Q2 and actively turns off the high-side MOSFET Q2 independent of load current. According to this embodiment, the device is a bipolar junction transistor (BJT) Q3 having a base coupled to a node between the cathode of rectifier diode D4 and the anode of rectifier diode D2, an emitter coupled to the gate of the high-side MOSFET Q2 at a node between the cathode of rectifier diode D2 and resistor R2 and a collector coupled to a node between the series connection of the high-side and low-side MOSFETs Q1 and Q2. A resistor R3 is coupled between the base and collector of BJT Q3.

During turn on of the cascode switch 600, pulse source V1 turns on the low-side MOSFET Q1 and rectifier diodes D2 and D4 begin to conduct and charge the input capacitance of the high-side MOSFET Q2 through resistor R2. The high-side MOSFET Q2 turns on in response. BJT Q3 is off because the base-emitter voltage (Vbe) of Q3 is negative.

During turn off of the cascode switch 600, pulse source V1 turns off the low-side MOSFET Q1 and Vds of Q1 begins to rise. This in turn causes diodes D2 and D4 to become reverse biased and BJT transistor Q3 to conduct and discharge Cgs of the high-side MOSFET Q2, turning off Q2. The voltage on both MOSFETs Q1 and Q2 rises simultaneously. The turn off time of the high-side MOSFET Q2 does not depend on the load current according to this embodiment, meaning that there may still be a slight dependence of the turn off time on load current e.g. as in the case of a single MOSFET. The circuit topology does not result in any additional dependency of turn off time on load current.

FIG. 7 illustrates an embodiment of a cascode switch 700 which is similar to the embodiment shown in FIG. 6, however an additional power transistor Q4 is coupled in series with the first and second power transistors Q1 and Q2 so that a voltage applied to the load is distributed across all of the series connected power transistors. An additional BJT Q5 is also provided in the same configuration as BJT Q3 for actively turning off power transistor Q4 independent of the load current. In more detail, BJT Q5 has a base coupled to a node between the cathode of rectifier diode D4 and the anode of rectifier diode D1, an emitter coupled to the gate of power transistor Q4 at a node between the cathode of rectifier diode D1 and resistor R4 and a collector coupled to a node between the series connection of power transistors Q4 and Q2. Resistor R5 is coupled between the base and collector of BJT Q5.

During turn on of the cascode switch 700, pulse source V1 turns on MOSFET Q1 and rectifier diodes D5 and D2 begin to conduct and charge the input capacitance of power transistor Q2 through resistor R2. Transistor Q2 turns on in response. BJT transistor Q3 is off because the Vbe of Q3 is negative. Rectifier diodes D4 and D1 also begin to conduct and charge the input capacitance of power transistor Q4 through resistor R4, and Q4 turns on in response. BJT transistor Q5 is also off because the Vbe of Q5 is negative.

During turn off of the cascode switch 700, pulse source V1 turns off MOSFET Q1 and Vds of Q1 starts to rise. This in turn causes rectifier diodes D2 and D5 to become reverse biased, and BJT transistor Q3 to conduct and discharge the Cgs of MOSFET Q2. Transistor Q2 turns off in response. Similarly, rectifier diodes D1 and D4 become reverse biased and BJT transistor Q5 conducts and discharges the Cgs of MOSFET Q4, and Q4 turns off. The voltage rises on all three MOSFETs Q1, Q2 and Q4 simultaneously. As with the embodiment illustrated in FIG. 6 the turn off time of MOSFETs Q1, Q2 and Q4 does not depend on load current according to this embodiment, meaning that there may still be a slight dependence of the turn off time on load current e.g. like for a single MOSFET. Also, the circuit topology does not result in any additional dependency of turn off time on load current.

During turn off and because of component tolerances, one of the power transistors included in a cascode switch can achieve maximum voltage before the other one(s). This transistor enters avalanche mode until the other power transistor turns off. The duration of avalanche is very small e.g. less than 20 ns, and avalanche energy dissipated on the power transistor is typically well below the avalanche energy of the transistor. However if the power transistor does not have avalanche capability, a voltage limiter can be introduced.

FIG. 8 illustrates an embodiment of a cascode switch 800 which is similar to the embodiment shown in FIG. 6, with the addition of voltage limiter circuitry. According to one embodiment, a first voltage limiter includes rectifier diode D6 and TVS diode D1 connected in parallel with MOSFET Q1. A second voltage limiter includes rectifier diode D7 and TVS diode D8 connected in parallel with MOSFET Q2. TVS diodes D1 and D8 function as high voltage Zener diodes to protect MOSFET Q1 and Q2, respectively. The first and second voltage limiters protect MOSFETs Q1 and Q2 by preventing or at least minimizing avalanche conditions. According to another embodiment, rectifier diode D5 is provided instead of diodes D6, D1, D7 and D8. Rectifier diode D5 has a cathode coupled to the load source V2 and an anode coupled to the source of low-side MOSFET Q1, and protects Q1 from avalanche conditions. In this case, the low-side MOSFET Q1 sustains the input voltage and the high-side MOSFET Q2 sustains the transformer reflected voltage and spikes caused by leakage inductance.

Further enhancements can be made to the cascode switches previously described herein. For example, lossless switching may be provided by ensuring the transformer reflected voltage is equal to the load input voltage and the cascode switch is turned on when the voltage equals or is close to zero.

FIG. 9 illustrates an embodiment of a cascode switch 900 which is similar to the embodiment shown in FIG. 6. Compared to FIG. 6, the cascode switch 900 shown in FIG. 9 has an additional capacitor C1 coupled in parallel with the series connected power MOSFETs Q1 and Q2 and a resistor R4 which couples pulse voltage source V1 to the anode of rectifier diode D4. Resistor R4 enables asymmetric switching of the cascode switch 900 i.e. different turn on and off periods. When MOSFETs Q1 and Q2 turn off, current moves from MOSFETs Q1 and Q2 to capacitor C1. The load current charges capacitor C1, and there are no losses on the cascode switch 900 because capacitor C1 is charged by the load current until the voltage reaches a certain value, when secondary diode D3 begins to conduct. When all energy from the transformer (P1/S1) is released to the secondary winding S1, diode D3 stops conducting and begins to oscillate between the switch capacitance (MOSFETs+C1) and the transformer inductance. When the voltage on the cascode switch 900 reaches zero, the switch 900 turns on and the turn off energy stored in capacitor C1 discharges to the load input source V2.

Terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are not intended to be limiting. Like terms refer to like elements throughout the description.

As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.

It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof. 

What is claimed is:
 1. A cascode switch, comprising: a first power transistor configured to be coupled to a load; a second power transistor coupled in series with the first power transistor so that the second power transistor is between ground and the first power transistor and the first power transistor is between the second power transistor and the load; and wherein the second power transistor is operable to switch on and off responsive to a pulse source coupled to a gate of the second power transistor, wherein the first power transistor is operable to switch on and off responsive to the same pulse source as the second power transistor or a DC source coupled to a gate of the first power transistor, and wherein the first and second power transistors form a cascode switch operable to switch a load current flowing through the load.
 2. A cascode switch according to claim 1, wherein the first and second power transistors are MOSFETs.
 3. A cascode switch according to claim 1, wherein the first and second power transistors are IGBTs.
 4. A cascode switch according to claim 1, further comprising a rectifier diode coupling the pulse source or the DC source to the gate of the first power transistor.
 5. A cascode switch according to claim 1, further comprising an energy recovery circuit operable to store energy from the load during switching off of the first and second power transistors and reuse the stored energy to power the load during a subsequent switching cycle of the first and second power transistors.
 6. A cascode switch according to claim 1, further comprising a device coupled to the gate of the first power transistor and operable to actively turn off the first power transistor independent of the load current.
 7. A cascode switch according to claim 6, further comprising a rectifier diode coupling the pulse source or the DC source to the gate of the first power transistor, and wherein the device is a bipolar junction transistor having a base coupled to a cathode of the rectifier diode, an emitter coupled to the gate of the first power transistor and a collector coupled to a node between the series connection of the first and second power transistors.
 8. A cascode switch according to claim 7, wherein the rectifier diode is operable to become reverse biased responsive to the pulse source or the DC source turning off, the bipolar junction transistor is operable to conduct current when the rectifier diode is reverse biased, and the first power transistor is operable to switch off responsive to the bipolar junction transistor conducting current.
 9. A cascode switch according to claim 6, further comprising: an additional power transistor coupled in series between the first and second power transistors so that a voltage applied to the load is distributed across all of the series connected power transistors; and an additional device coupled to a gate of the additional power transistor and operable to actively turn off the additional power transistor independent of the load current.
 10. A cascode switch according to claim 1, further comprising: a first voltage limiter coupled in parallel with the first power transistor and operable to limit the voltage across the first power transistor; and a second voltage limiter coupled in parallel with the second power transistor and operable to limit the voltage across the second power transistor.
 11. A cascode switch according to claim 1, further comprising a rectifier diode having a cathode coupled to an input source of the load and an anode coupled to ground.
 12. A cascode switch according to claim 1, further comprising a capacitor operable to charge via the load current when the first and second power transistors are switched off and discharge to an input source of the load responsive to the first and second power transistors subsequently switching on.
 13. A circuit, comprising: a load operable to draw a load current; a cascode switch operable to switch the load current, the cascode switch comprising a first power transistor coupled to the load and a second power transistor in series between ground and the first power transistor, a gate of the second power transistor being coupled to a pulse source and a gate of the first power transistor being coupled to the same pulse source as the second power transistor or a DC source; and a rectifier diode coupling the pulse source or the DC source to the gate of the first power transistor.
 14. A circuit according to claim 13, further comprising a device operable to actively turn off the first power transistor independent of the load current.
 15. A cascode switch, comprising: a first power transistor configured to be coupled to a load; a second power transistor in series with the first power transistor so that the second power transistor is between ground and the first power transistor and the first power transistor is between the second power transistor and the load, the first and second power transistors collectively operable to switch a load current flowing through the load; and a device coupled to a gate of the first power transistor and operable to actively turn off the first power transistor independent of the load current.
 16. A cascode switch according to claim 15, wherein the device is a bipolar junction transistor.
 17. A cascode switch according to claim 15, wherein the device is operable to conduct current when a source coupled to the gate of the first power transistor is switched off, and the first power transistor is operable to switch off responsive to the device conducting current.
 18. A cascode switch according to claim 15, further comprising: an additional power transistor coupled in series between the first and second power transistors so that a voltage applied to the load is distributed across all of the series connected power transistors; and an additional device coupled to a gate of the additional power transistor and operable to actively turn off the additional power transistor independent of the load current.
 19. A cascode switch according to claim 15, further comprising: a first voltage limiter coupled in parallel with the first power transistor and operable to limit the voltage across the first power transistor; and a second voltage limiter coupled in parallel with the second power transistor and operable to limit the voltage across the second power transistor.
 20. A cascode switch according to claim 15, further comprising a rectifier diode having a cathode coupled to an input source of the load and an anode coupled to ground.
 21. A cascode switch according to claim 15, further comprising a capacitor operable to charge via the load current when the first and second power transistors are switched off and discharge to an input source of the load responsive to the first and second power transistors subsequently switching on.
 22. A method of switching a load via a cascode switch having a first power transistor coupled to the load and a second power transistor in series between ground and the first power transistor so that the first power transistor is between the second power transistor and the load, the method comprising: switching on and off the second power transistor of the cascode switch via a pulse source coupled to a gate of the second power transistor; and switching on and off the first power transistor of the cascode switch via the same pulse source as the second power transistor or via a DC source coupled to a gate of the first power transistor, wherein the switching on and off of the first and second power transistors causes the cascode switch formed by the first and second power transistors to switch a load current flowing through the load.
 23. A method according to claim 22, further comprising: storing energy from the load during switching off of the first and second power transistors; and reusing the stored energy to power the load during a subsequent switching cycle of the first and second power transistors.
 24. A method according to claim 22, further comprising actively turning off the first power transistor independent of the load current.
 25. A method according to claim 22, further comprising: limiting the voltage across the first power transistor to prevent avalanche breakdown in the first power transistor; and limiting the voltage across the second power transistor to prevent avalanche breakdown in the second power transistor.
 26. A method according to claim 22, further comprising: charging a capacitor via the load current when the first and second power transistors are switched off; and discharging the capacitor to an input source of the load responsive to the first and second power transistors subsequently switching on.
 27. A method of switching a load via a cascode switch having a first power transistor coupled to the load and a second power transistor in series between ground and the first power transistor so that the first power transistor is between the second power transistor and the load, the method comprising: switching on the first and second power transistors of the cascode switch; and switching off the first and second power transistors of the cascode switch independent of the load current, wherein the switching on and off of the first and second power transistors causes the cascode switch formed by the first and second power transistors to switch a load current flowing through the load. 